Back to NeuroanatomyThe cerebrum has two hemispheres separeted by the longitudinal cerebral fissure, into which the falx cerebri projects. The commissures ( anterior, posterior and corpus callosum ) are responsible for the connections between the hemispheres. Each hemisphere has three surfaces, three poles, three margins and six lobes.
Surfaces
The cerebrum has three surfaces, the superolateral, the medial and the inferior. The superolateral or external surface is convex because it is adapted to the curve of calvaria. The medial or internal surface is flat and vertical, and separeted from each other by the longitudinal cerebral fissure. The inferior surface is irregular and divided by the stem of the lateral sulcus into two portions, an anterior or orbital which is smaller and located over the orbital roof and, a posterior or tentorial located over the middle cranial fossae and over the tentorium cerebelli.
Poles
The cerebrum has three poles, the frontal, the temporal and the occipital. They correspod to the tip of their lobes.
Margins
there are three margins in the cerebrum, superior, inferolateral and inferomedial. The superior margin separates the superolateral from the medial surface, the inferolateral margin is located between the superolateral and the inferior surfaces, while the inferomedial margin separates the medial from the inferior surface.
Lobes of The Superolateral Surface
To divide the external surface in lobes we need to find four landmarks, the central sulcus, the lateral sulcus, the parietooccipital sulcus and the preoccipital notch.
The central sulcus ( sulcus of Rolando ) is located about 1cm behind the midpoint between the frontal and occipital poles in the superior margin. It runs obliquely downward and forward ending just above the middle of the lateral sulcus. In the medial surface this sulcus ends in the paracentral lobule.
The lateral sulcus ( fissure of Sylvius ) is the most important and deepest. It has a lenght of 11cm and begins in the inferior surface laterally to the anterior perforated substance. It runs initially outward and forward between the frontal and temporal lobes, reaching the superolateral surface it begins to run backward and upward. It has three rami in the superolateral surface, the anterior, ascending and posterior rami. The latter is the main part of the sulcus in the superolateral surface, whereas the anterior and ascending rami run for a short distance into the inferior frontal gyrus.
The parieto-occipital sulcus is easily recognizable on the medial surface of the cerebrum. It runs from the calcarine sulcus to the superior margin of the hemisphere, cutting this margin 5cm before the occipital pole. In the superolateral surface it extends for a short distance.
The preoccipital notch is a shallow landmark located in the inferolateral margin about 5cm before the occipital pole. It is formed by the petrous portion of the temporal bone.
The frontal lobe is located in front of the central sulcus and superior to the lateral sulcus. It has four gyri and three sulci on the superolateral surface. There are two anteroposterior sulci running parallel to the superior margin, the superior and inferior frontal sulci. The posterior end of each one give off two branches, an ascending and a descending rami. They join and form the precentral sulcus, which is parallel to the central sulcus.
The three frontal sulci divide the frontal lobe in four gyri. Between the central and precental sulci is the precentral gyrus which is the primary motor area of the cerebral cortex. The superior frontal gyrus is located above the superior frontal sulcus. The middle frontal gyrus is between the superior and inferior frontal sulci. It is larger than the others frontal gyri and has some longitudinal and transverse sulci, what make difficulties to its recognition. The inferior frontal gyrus is located below to the inferior frontal sulcus. It is divided from before backward into orbital, triangular and opercular portions by the anterior and ascending rami of the lateral sulcus. In the left hemisphere of most people the triangular and opercular portions form the motor speech area of Broca. Damage in the Broca`s area results in the inability to say what is thought ( motor aphasia ).
The parietal lobe in the external surface is limited anteriorly by the central sulcus, posteriorly by an imaginary line which runs from the parieto-occipital sulcus to the preoccipital notch, superiorly by the superior margin and inferiorly by the lateral sulcus.
Running parallel to the central sulcus is the postcentral sulcus and between them is the postcentral gyrus which receives the general sensation from the contralateral side of the body, forming the primary sensory area of the cortex. From the midpoint of the postcentral sulcus runs backward the intraparietal sulcus divinding the parietal lobe into superior and inferior parietal lobules, and this sulcus usually reaches the occipital lobe. The intraparietal sulcus gives the intermediate sulcus of Jensen,a descending branch which divides the inferior parietal lobule into two portions, the supramarginal and the angular gyri.
The superior temporal gyrus ends in the parietal lobe and then is divided into two portions an anterior and a posterior; the former surrounds the upturned end of the lateral sulcus and is called supramarginal gyrus, while the posterior portion joins the middle temporal gyrus, surrounding the upturned end of the superior temporal sulcus and is called angular gyrus.
The temporal lobe is limited superiorly by the lateral sulcus and the parietal lobe and posteriorly by occipital lobe. This lobe is divided by the superior and inferior temporal sulci into superior, middle and inferior gyri. The superior temporal sulcus begins about 9mm behind the temporal pole, running backward and upward to ends in the parietal lobe. It is a deep sulcus which rarely is interruped by anastomotic curvatures. The inferior temporal sulcus is not so deep and is usually interruped by anastomotic curvatures. The superior temporal gyrus forms the superior face of the temporal lobe which lies inside the lateral sulcus.
The occipital lobe is located behind the imaginary line which runs from the preoccipital notch to the parieto- occipital sulcus. The lobe has a transverse occipital sulcus which is the continuation of the intraparietal sulcus, and has a inconstant sulcus, the lunate sulcus. Some authors divide the occipital lobe into superior, middle and inferior gyri, and the transverse occipital and the lunate sulci representing a superior and a inferior occipital sulci respectively.
The Insula, or Island of Reil can be visualized separating the two margins of the lateral sulcus. It has a triangular shape, with the apex or limen of the insula located anterior and inferiorly, and the base is located superiorly. The insula is surrounded by the circular sulcus of insula and divided into anterior and posterior portions by the central sulcus of insula which is easily visualized. In front of the central sulcus of insula is the three short gyri which arise from the insula pole. Behind the central sulcus of insula is the two ( sometimes one ) long gyri of insula. Behind the long gyri is the retroinsular portion and the correspnded ramus of the circular sulcus. The transverse temporal gyri are located in the retroinsular portion.
The Medial Surface
The first aspect to pay our attention in the medial surface is the corpus callosum which is divided into four portions. Anteriorly the genu of corpus callosum has a downward and backward curvature which is continue with the rostrum of corpus callosum. Posteriorly the corpus callosum ends in the splenium. Located between the genu and the splenium is the trunk of corpus callosum. From genu to splenium the lenght is of 7 or 8cm.
The rostrum ends into the anterior commissure, which continues downward with the lamina terminalis and this reaches the optic chiasm. The lamina terminalis is the anterior bound of the third ventricle.
Arising in the medial surface, below of the splenium and trunk of corpus callosum is the fornix, it is a simetrical structure divided into three portions. The body of fornix is located in the roof of third ventricle and is formed by the meet of the fornix of each hemisphere and below that you can visualize the choroid plexus of the third ventricle. Anteriorly and posteriorly the body of the becomes separeted. Anteriorly as the columns of fornix, curving downward to the mamillary bodies and posteriorly as the crura of fornix. Between the crura is the commissure of fornix, which was called by the ancient anatomists as cerebral vulva.
The septum pellucidum is located between the corpus callosum and the fornix. They are two thin sheets delimiting a narrow space, separating the lateral ventricles. Each sheet is located in a hemisphere and the narrow space between them is not part of the third ventricle.
The sulcus of corpus callosum begins beneath the rostrum and runs through the entire lenght of corpus callosum above it, to ends in continuity with the hippocampal sulcus. The former sulcus separates the corpus callosum from the cingulate gyrus and this gyrus is continues with the septal area, under the rostrum. This area is composed by the subcallosum area and the paraterminal gyrus. The cingulate gyrus is continues with the isthmus of cingulate gyrus behind the splenium, and this is continues with the parahippocampal gyrus.
The cingulate sulcus begins in the septal area running parallel to the sulcus of corpus callosum, and ends in two ramus, the marginal ramus which runs to the superior margin of the cerebral hemisphere, and the subparietal sulcus which separates the cingulate gyrus from the precuneus. Arising from the cingulate sulcus is the paracentral sulcus which runs to the superior margin of the hemisphere.
Anterior to the paracentral sulcus is the medial portion of the superior frontal gyrus. Between the paracentral sulcus and the marginal ramus of cingulate sulcus is the paracentral lobule which has this name because the central sulcus ends in its superior margin. The postcentral and precentral gyri are continuing into the paracentral lobule.
The calcarine sulcus begins below the splenium and ends near of the occipital pole. The parieto-occipital sulcus begins in the supero-lateral surface to ends in the calcarine sulcus. The precuneus is located between the marginal ramus of cingulate sulcus and the parieto-occipital sulcus and between this later and the calcarine sulcus is the cuneus.
The medial portion of the superior frontal gyrus and the part of the paracentral lobule anterior to the central sulcus are portions of the frontal lobe. The precuneus and the part of the paracentral lobule posterior to the central sulcus pertains to the parietal lobe. The cuneus is part of the occipital lobe.
The inferior surface has the frontal portion, anterior and the occipitotemporal portion, the former lies in the roof of the orbit, and the later lies over the middle cranial fossa and over the tentorium cerebelli.
The Frontal Lobe
In the inferior surface it has medially the gyrus rectus which is continues with the superior frontal gyrus anteriorly and medially. Laterally to that is the olfactory sulcus which is a deep long sulcus with a antero-posterior orientation. In this sulcus we could visualize the olfactory tract and bulb. The olfactory nerves make connections in the olfactory bulb. The rest of the frontal lobe in the inferior surface is formed by the very irregular orbital sulci and gyri.
The olfactory tract ends dividing into three olfactory striae ( medial, intermediate and lateral ) which delimits a triangular area, the olfactory trigone, delimiting with the optic tract posteriorly the anterior perforated substance.
The Temporal and Occipital Lobes
When we study this part of the cerebrum for the first time it looks difficult, but it is a simple area of the cerebrum. It has three principal sulci with a longitudinal orientation and are from the medial to the lateral margin the hippocampal sulcus, the collateral sulcus and the occipitotemporal sulcus.
The occipitotemporal sulcus and the inferior temporal sulcus delimit the inferior temporal gyrus. The occipitotemporal lateral gyrus is located between the occipitotemporal and the collateral sulci. The occipitotemporal medial gyrus is located between the collateral sulcus, laterally and the calcarine sulcus medially. The parahippocampal gyrus which is the anterior continuation of the occipitotemporal medial gyrus and of the isthmus of the cingulate gyrus is delimited laterally by the collateral sulcus and medialy by the hippocampal sulcus. The anterior portion of the parahippocampal gyrus makes a medial curvature to continues with the uncus. Separating this anterior portion of the parahippocampal gyrus from the occipitotemporal lateral gyrus is the rhinal sulcus.
The limbic lobe is formed by the cingulat*e and parahippocampal gyri, the isthmus of the cingulate gyrus and several additional structures, as the dentate gyrus, amygdaloid body, hypothalamus, septal area, and anterior nuclei of the thalamus. In the inferior surface the imaginary line from the preoccipital notch to the parieto-occipital sulcus delimits the occipital and temporal lobes, but the parahippocampal gyrus is a portion of the limbic lobe, not of the temporal lobe.
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